Chapter 3, An Inexpensive Vertical Seismometer for Classroom Demonstrations: Step by Step Construction

Developed for the Mid America Earthquake Center Workshop

at Georgia Tech, June 22-25, 1998

 

Introduction

In the previous two sections we have described the theory of operation and given a general description of the low-cost vertical seismometer. In Chapter 2 we showed a picture of the parts used to construct the long period vertical seismometer. In this section we provide step by step instructions for putting the seismometer together and add comments on how to make individual components.

Assembly

1) Mount the U-bolt first. Put one nut on each side of the U-bolt and screw on so that approximately 1 1/4" if threads extend beyond the nut. Place the cross plate on the U-bolt and insert ends of U-bolt into the wood base. Secure in place with two nuts on the bottom, adjusting nuts as necessary to provide equal extension of about 1/8 inch below the nuts on the bottom. The equal extension length of 1/8 inch is important because these form two of the three legs of the seismometer and determine the side-to-side level of the seismometer. The ends of the U-bolt may, if needed, be filed smooth to eliminate sharp edges that might mar a smooth table surface. Smooth ends will also enhance stability. If assembling your own parts, measure the width of the U-bolt to be sure that the holes are drilled at the proper separation. The U-bolt should be about 4 A from the end of the board.

 

 

2) The leveling screw provides the third leg for the base plate. Place the leveling screw mount (a 3/8" threaded insert) into the bottom of the base plate. It is advisable to use a vice or c-clamp to push the leveling screw mount into position because a hammer could damage the base plate. Place a spare nut on the leveling bolt. Use a file to round off the end of the leveling bolt that will rest on the table. The nut can then be used to clean out the threads after filing. The spare nut will also serve as a locking nut once proper balance is determined. Insert the leveling bolt into the screw mount from the top of the base, so that the rounded end will rest on the table. The head of the screw and the nut are on top so that they are accessible for level adjustment. As an alternative to filing, the end of the screw may be rounded with a drop of epoxy to allow it to turn easily on a flat surface. The leveling screw top is shown in the picture to the below next to the sensor mount.

3. Mounting the sensor block. First glue the sensor (the light sensitive resistor) in the back of the 3/4 A hole in the sensor block so that the light sensitive side faces out through the 3/4"hole. Pull the wires through the smaller hole in back and bend them to hold in place while the glue dries. These wires are then connected to the electronic measuring device for observation of movement. Also, for the convenience of attaching and removing wires to other instruments, they may be attached to two screws (not provided) set into the sensor block.

4) Mount the sensor block to the base with 1 1/4" #6 wood screws from the bottom of the base plate, so that the large hole in the sensor faces toward the centerline of the base. We used epoxy to place the sensor in the back of the large hole so that the sensor faces toward the center of the base. The wires may be bent toward the top where screw posts may be added to facilitate electrical connections. In our demonstrations we simply attached clip leads to the sensor wires. Electrical connecting posts would be advisable to avoid damage in rough handling.

5) Insert the eye bolt into the spring support and using bolts mount the spring support on the baseboard. Do not put on too tight because the settings may need to be adjusted when setting the free period. A second nut should be installed on the "eye" end of this bolt to serve as a locking nut after adjustments of the spring are completed.

 

6) Using the bolts, mount the two 2 A wide metal tabs onto the travel limit bracket. Leave loose so that adjustment is easy for installation. In final adjustment, these will be secured to prevent lever arm movement outside the linear range of the position sensor. With two wood screws, mount the travel limit bracket onto the baseboard. The travel limit bracket can be glued in place to give it more stability after its correct position is verified.

7) Hinge Assembly. The hinge pieces in this seismograph were made from the same aluminum edge bars as were used for the lever arm. However, any aluminum stock with a right angle would work. There is a top and bottom to the hinge. The top has openings to allow the placement of the U-bolt in position. The bottom has a notch for insertion of the lever arm. Both have holed drilled for placement of the bolts to secure the hinges. Use the 1/4 " long machine screws, a washer and nut to connect the two hinge pieces together. Place the washer on the bolt first and then place the shim on the bolt before inserting into the angle bracket. The thin hinge shims included in these models were made from the sidewalls of aluminum drink cans. The machine screws fit tightly into the holes in the angle pieces. However, the shims fit loosely in order to accommodate slight errors in placing the holes. Take care not to bend or twist the metal shims of the hinge. The metal is brittle and could tear. We cut 3/8" by 9/8" pieces and drilled 3/16 diameter holes at a separation of 5/8". In order to drill the hole without damaging the shim, we clamped it between two pieces of metal. Note that on the top piece we used a 3/4' bolt in order to attach the brace on the back that holds the hinge to the U-bolt. The top angle brace is the one with the holes for placement on the U-bolt. When all the shims are mounted with screws and nuts, secure them carefully in place. The hinges are held in place with screws and washers bolted from the rear. The screws should be tightened carefully so as not to twist or bend the hinges. The screws are #6 /32 (1/4 or 3/8 inch long) and the holes are drilled to 1/8 inch for a tight fit. The nuts are put on the back to tighten down after all are aligned properly. Put the small machine screws on first because the longer screws and clamping bar cover one of the nuts and make it very difficult to assemble after the longer screws are in place.

8) Mount the hinge on the U-bolt and secure with the two longer machine screws used to attach the shims and the 2A clamping bar.

9) Bolt the lever arm to the hinge using a 1 1/4 A machine screw and nut.

 

 

 

 

 

 

10) Attach a large gauge electrical wire by placing it in the holes provided at the end of the lever arm. This wire may be bent to support a light shield to cover the sensor. Bend the wire in a AL@ shape to allow attachment of a light shutter for the position sensor. This shutter may be made of any material that will shadow the light sensor (black electrical tape, metal foil, etc). To make this position sensor more effective, direct a light (such as a flash light) across the light shutter to the light sensitive sensor. This will control the light falling upon the sensor as the seismometer arm moves up and down. The light changes on the sensor will produce a similar change in its output signal.

11) Check to see that the lever arm is secure in its constraint. The spring can then be attached as shown in the picture on the cover.

 

12) Place the weight on the lever arm to balance the spring, being careful to find the position that balances the lever arm in the middle of the travel limits. The spring and mass supplied with this kit have already been tested so that the mass should balance near the long end of the lever arm. When a good position is found, glue the weight in place (we do not want it to fall off). Alternately, a hole may be drilled in the lever arm and a screw used to hold the lead in place. Large paper clips may be used for fine balance adjustment.